The rapid spanning tree protocol is a variant of the spanning tree protocol that is used to ensure loop free data forwarding in a layer 2 (Open Systems Interconnect Reference Mode-OSI) network topology. The rapid spanning tree protocol is used to determine a path for traffic to be forwarded across a network to ensure that the traffic does not loop back to the sending network element. However, the use of the rapid spanning tree protocol is complicated by the presence of virtual local area networks within the topology of the larger network. The virtual local area networks have a distinct topology from the overall network and a rapid spanning tree developed based on the entire network may not be efficient or guarantee the proper transmission of traffic for any particular virtual local area network within the network. The use of a single spanning tree at each node of the network is referred to as mono spanning tree.
A rapid spanning tree can be maintained by a network element at a port level and each port where all virtual local area networks are discarding is blocked. However, this prevents the use of alternate paths for different virtual local area networks, resulting in inefficient use of overall network bandwidth and resources.
One solution to the problem posed by the combination of virtual local area networks and the rapid spanning tree protocol is to create a rapid spanning tree protocol instance for each virtual local area network. However, this solution introduces a problem of scale. On any given network there can be a large number of virtual local area networks operating within its topology. Storing and maintaining a separate spanning tree instance for each virtual local area network can be computationally intensive and also require significant storage space and bandwidth.